1. Field of the Invention
The present invention relates to a method of manufacturing a rare earth magnet.
2. Description of Related Art
Rare earth magnets made from rare earth elements are called permanent magnets and are used for driving motors of hybrid vehicles, electric vehicles, and the like, as well as motors included in hard disks and MRIs.
As an index indicating magnet performance of these rare earth magnets, for example, remanent magnetization (remanent magnetic flux density) and coercive force may be used. Along with a decrease in the size of a motor and an increase in current density, the amount of heat generation increases; and thus the demand for high heat resistance has further increased in rare earth magnets to be used. Accordingly, one of the important research issues in this technical field is how to hold the coercive force of a magnet when being used at a high temperature. A Nd—Fe—B-based magnet which is a rare earth magnet widely used in a vehicle driving motor will be described as an example. In this Nd—Fe—B-based magnet, an attempt to increase the coercive force thereof has been made, for example, by refining crystal grains, by using an alloy composition having a large amount of Nd, or by adding a heavy rare earth element such as Dy or Tb having high coercive force performance.
Examples of the rare earth magnets include commonly-used sintered magnets in which a grain size of crystal grains constituting a structure thereof is about 3 μm to 5 μm; and nanocrystalline magnets in which crystal grains are refined into a nano grain size of about 50 nm to 300 nm.
In order to improve the coercive force among magnetic properties of such a rare earth magnet, PCT International Publication WO 2012/008623 discloses a method in which, for example, a Nd—Cu alloy or a Nd—Al alloy is diffused and infiltrated into a grain boundary phase as a modified alloy containing a transition metal element or the like and a light rare earth element to modify the grain boundary phase.
Since the modified alloy containing a transition metal element or the like and a light rare earth element does not contain a heavy rare earth element such as Dy, the modified alloy has a low melting point, is melted even at about 700° C., and can be diffused and infiltrated into the grain boundary phase. Accordingly, in the case of nanocrystalline magnets having a grain size of about 300 nm or less, it can be said that the above processing method is preferable because coercive force performance can be improved by modifying the grain boundary phase while suppressing the coarsening of crystal grains.
However, when the Nd—Cu alloy or the like is diffused and infiltrated into the grain boundary phase, in order for the Nd—Cu alloy or the like to be diffused and infiltrated into the center of the magnet, it is necessary that the infiltration amount of the Nd—Cu alloy or the like or the heat treatment time be increased.
In this case, the Nd—Cu alloy itself is a non-magnetic alloy, and thus when the infiltration amount of the Nd—Cu alloy or the like to be diffused and infiltrated is increased, the content of a non-magnetic alloy in the magnet is increased, which leads to a decrease in the remanent magnetization of the magnet. In addition, an increase in the infiltration amount of the Nd—Cu alloy or the like causes an increase in material cost.
In addition, the diffusion and infiltration of the Nd—Cu alloy or the like using a long-term heat treatment leads to an increase in the manufacturing time and cost of a magnet.
On the other hand, instead of the diffusion and infiltration of the modified alloy, PCT International Publication WO 2012/036294 discloses a method of manufacturing a rare earth magnet in which a heat treatment is performed on a rare earth magnet precursor subjected to hot deformation processing at a temperature, which is sufficiently high for causing a grain boundary phase to be diffused or flow and is sufficiently low for preventing the coarsening of crystal grains, such that a grain boundary phase concentrated on triple points of crystal grains is sufficiently infiltrated into a grain boundary other than the triple points to cover each crystal grain, thereby improving coercive force performance. Such a heat treatment may be also called an optimal heat treatment or an aging treatment.
The low temperature during the heat treatment defined herein is about 700° C. at the highest as in the case of PCT International Publication WO 2012/008623. In order to cause the grain boundary phase to be diffused or flow at such a low temperature, a rare earth magnet composition is represented by, for example, Nd15Fe77B7Ga, and a rare earth magnet is manufactured from a composition material having a Nd-rich grain boundary.
However, in the manufacturing method disclosed in PCT International Publication WO 2012/036294, the modified alloy is not diffused and infiltrated. Therefore, in terms of the coercive force performance of, for example, a surface region (outer peripheral region) of a magnet, deterioration in coercive force performance is inevitable as compared to the case of the manufacturing method in which the modified alloy is diffused and infiltrated.
Therefore, a simple combination between the above-described two techniques may be considered, the techniques including: the technique disclosed in PCT International Publication WO 2012/008623, that is, the manufacturing method in which the modified alloy is diffused and infiltrated; and the technique disclosed in PCT International Publication WO 2012/036294, that is, the manufacturing method in which a grain boundary phase is caused to, for example, flow by a heat treatment at a low temperature. According to a manufacturing method which is a combination of the above related techniques, it is considered that the coercive force of a surface region of a magnet can be improved by the diffusion and infiltration of the modified alloy, and the coercive force of a center region of the magnet can be improved by the flow or the like of a grain boundary phase.
However, PCT International Publication WO 2012/008623 and PCT International Publication WO 2012/036294 mainly focus on the improvement of coercive force performance and do not have a configuration relating to the above-described problem, that is, a decrease in remanent magnetization which is caused when the infiltration amount of the modified alloy is excessively large. Therefore, with the simple combination between the techniques disclosed in PCT International Publication WO 2012/008623 and PCT International Publication WO 2012/036294, a method of manufacturing a rare earth magnet which is superior in both coercive force performance and magnetization performance cannot be obtained.